1. Technical Field
The present invention relates to a wire bonding joint structure of a joint pad, a method for preparing the same, and a printed circuit board including the same.
2. Description of the Related Art
Due to high integration of electronic parts, a technology of treating a surface of a printed circuit board (PCB) is diversified. Recently, as a demand for thin and highly integrated PCB products is increased, the surface treatment technology has been changed from an electrolytic Ni/Au surface treatment technology to an electroless surface treatment technology capable of easily implementing tailess so as to implement process simplification, noise free, or the like.
In particular, electroless Ni/Au (hereinafter, referred to as ENIG) used in the related art and electroless Ni/Pd/Au (hereinafter, referred to as ENEPIG) recently interested have good solder joint reliability and wire bonding reliability and as a result, has been used for various fields in addition to a package substrate. However, as wirings are miniaturized so as to highly integrate electronic parts, problems of the technologies for treating the surface of the circuit board according to the related art start to appear.
Next, FIGS. 1A and 1B each are a diagram showing a cross section when devices are connected to each other using a wire bonding method of the related art and a diagram viewed from the top.
Referring to FIGS. 1A and 1B, a polymer resin layer 20 that is formed in a portion except for a copper layer 10 on a printed circuit board serves as a resist for subsequent plating. As plating layers 30 for protecting the copper layer 10, Ni 31/Pd 32/Au 33 are formed. In addition, after the Ni/Pd/Au plating layers 30 are formed, metals are interconnected with each other by using an Au wire.
For example, FIGS. 2A to 2C are scanning electron microscope photographs of wire bonding pads connected by an Au wire according to the related art. When the electroless surface treatment has the electroless Ni/Au plating layers or the electroless Ni/Pd/Au plating layers, a thickness of each layer at the electroless Ni/Au is Ni at 3 μm or more and Au at 0.05 to 0.5 μm and a thickness of each layer at the electroless Ni/Pd/Au is Ni at 3 μm or more as the minimum, Pd at 0.05 to 0.3 μm, and Au at 0.05 to 0.5 μm.
The method of connecting a copper joint pad having the plating layers subjected to the electroless surface treatment and external devices by the Au wire bonding joints an electroless plating later on a Ni layer or a Ni—P layer that structurally has high hardness, such that it is not suitable for joining at the time of the wire bonding.
That is, the thickness of the Ni 31 is 3 μm or more as the minimum. In addition, the Ni layer having the thickness includes P (phosphorous) impurity of contents of about 6 to 9 wt %, such that the hardness of Ni is increased. When the hardness of Ni is increased, the Au wire is joined to the bonding pad and then, a deformation depth of the bonding pad is formed within about 1.0 μm by a wedge pressure in a final work that disconnects the wire bonding (see FIG. 2B). Therefore, the increase in the hardness may reduce wire bonding workability (see FIG. 2C).
Recently, an Au wire diameter is small and an Au thickness at a surface finish is also thin, due to the increase in the Au value, such that the wire bonding workability is reduced. Therefore, the improvements thereof are urgently needed.